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Technology update
Improving engine blisk manufacturing
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The ECM process has three stages: forging (lower right), rough cut (lower left), and finished blisk.
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Engineers at Sermatech International, Inc. have improved aircraft engine blisk manufacture through electrochemical machining (ECM). Sermatech-Lehr has been machining electrochemically for some time and has opened a new ECM facility in Cincinnati, OH for blisk production.
Aircraft engines and industrial gas turbines have traditionally used bladed compressor disks with individual airfoils anchored by nuts and bolts in a slotted central retainer. To reduce aerodynamic losses, weight, and complexity in the engine, some aircraft engine OEMs have replaced these disk assemblies with a one-piece bladed disk, called a blisk. One blisk can replace up to 120 parts or more in an engine.
Blisks can be CNC milled or manufactured by ECM. According to Sermatech-Lehr, there are several advantages to producing blisks by ECM instead of milling. In one engine plant, switching from traditional milling to ECM reduced dimensional deviations from 14 to 6%. ECM also enables manufacturers to fabricate airfoil shapes with pitch, twist, and camber optimized by computer-aided design. The machining of tall, thin blades, which are not possible using conventional milling, are made possible by the ECM process. In addition, ECM permits airfoils on blisks to be spaced 10 to 15% closer than milled components. The process also ensures smoother airfoils, with surface finishes measuring about 16 µin.
Electrochemically-machined compressor blisks similar to this one found in the F110 fighter engine feature both reduced mass and complexity compared to individually bladed rotors.
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The science of ECM has not changed since Sir Michael Faraday uncovered its basic principles in the 19th Century. In ECM, positively charged workpieces and negatively charged cathode tools are immersed in a circulating electrolyte solution. The saltwater electrolyte flushes positive ions dissolved away from the workpiece. According to Sermatech-Lehr, the process makes a precise mirror image of the cathode tool 30 to 50% faster than milling machines.
Sermatech-Lehr began producing compressor blisks in 1985 for the T700 helicopter engine. The company's blisks have also been found on the LM2500 gas turbine engine and the Boeing Rocketdyne RS68 rocket engine.
Frank Bokulich
Aerospace Engineering April 2000
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